1. Field of the Invention
This invention relates to an exposure control system of a camera having an automatic focusing (hereinafter referred to as AF) function.
2. Description of the Related Art
Cameras are generally arranged these days to perform automatic exposure (hereinafter referred to as AE) control and the AF control. A sharply focused picture can be taken by just pushing a shutter release button with camera directed to an object to be photographed. The AE control of the camera is performed either in a standard program mode in which an aperture value and a shutter time value are determined in accordance with a given program line in such a way as to have an exposure value obtained from the luminance of an object and the sensitivity of such photo sensitive means as a silver-halide film or the like on the basis of the output of a photometric sensor disposed within the camera, or in an aperture-priority program mode in which the photographer is allowed to set an aperture value as desired.
In the case of a single-lens reflex camera, for example, the AF control is performed in general by a phase difference detecting method in which a focusing lens is driven and controlled by detecting the light of the object obtained through a photo-taking lens by an AF sensor.
The camera of the kind using a silver-halide film generally has an allowable diameter of circle of confusion set at 35 .mu. or thereabout. Therefore, assuming that a photo-taking lens of F1.2 is mounted on the camera, for example, the allowable defocus degree of the camera becomes 35.mu..times.1.2=42 .mu.. The allowable defocus degree becomes larger accordingly as the aperture value of the photo-taking lens in use comes further away from the full aperture value of the lens.
Meanwhile, the focusing accuracy (AF accuracy) of the AF device operated by the above-stated phase difference detecting method is determined by the resolution of the AF sensor, the optical system, a processing algorithm, etc. The accuracy generally guaranteed nowadays is about 100 .mu. in terms of the defocus degree. With a silver-halide film, therefore, the AF accuracy cannot be guaranteed without setting the aperture at a value which is at least F2.8 or higher. Especially in the case of an electronic still camera which is arranged to use an image sensor such as a CCD as photo-sensing means, to convert the object's light into an image signal by photo-electric conversion and to record the video signal on a recording medium such as a magnetic disc or the like, the allowable diameter of circle of confusion is only about half of that of the camera using silver-halide film although it varies with the frequency band of the image signal. The aperture value at which the AF accuracy is warrantable for an electronic still camera is thus likely to greatly differ from the full aperture F number of the photo-taking lens.
Therefore, with an aperture value set at the full aperture F number or at a value close thereto either manually by the photographer in the aperture-priority program mode or automatically in the standard program mode according to the object's condition, a picture thus taken in the past has sometimes come out of focus even under a correct exposure condition.